Fiberglass reinforced plastic (FRP) ductwork is used extensively for fume exhaust of corrosive vapors in such industries as the semiconductor, pharmaceutical, waste water treatment plants, paper and pulp mills, college science buildings and plating shops. These ducts operate at pressures that are most often negative to atmosphere and must be joined in such a manner as to prevent leakage of air from the outside into the duct in order to prevent the waste of energy. In addition, such ducts are widely employed in industries where highly corrosive chemicals are used to process materials, and the interior of the duct is exposed to these harsh chemicals. Often such processes involve high humidities, and when acid or caustic-laden exhaust air enters the exhaust ductwork, a chilling factor occurs, and much condensate can accumulate on the interior of the duct. This condensate tends to puddle in the bottom of the duct and the chemicals in the condensate attack the joints used to connect duct sections to each other or to fittings. Such field joints are highly vulnerable to attack by these aggressive chemicals and can frequently cause failures in duct systems.
Heretofore, a number of different types of joints were used to make the field connections for FRP ductwork. Flanged duct sections have been used, but these require relatively precise dimensioning of product so that what is produced in a fiberglass manufacturer's shop will assemble properly in the field. Any errors in dimensioning of parts could cause considerable problems at the job site. A second prior method used for making FRP duct joints was a simple butt connection, wherein neither flanges or bells are used. Here, all duct and fittings are the same dimension and are merely butted together. The major advantage of butt joints is that the least care must be exercised in dimensioning products for installation in the field. However, some form of coupling device is necessary to hold adjoining duct sections together. A third method for making joints was to use duct sections with bell and spigot end joint portions wherein an enlarged bell is placed at one end of a duct or fitting, whose other end is a non-enlarged portion (the spigot) that fits into the bell portion of the next adjoining duct or fitting. The latter two joints, the bell and spigot joint, and the butt joint were generally bonded together by means of glass and resin, which provided a chemically made joint. This required that surfaces of the FRP parts be sanded on the exterior of the duct or fitting for a distance of 2" to 6" back from the centerline of the field joint. The reason for this is that almost all resins used to make FRP ducts provide an enamel-like finish which inherently rejects further bonding. Also, resin systems such as polyesters used for ductwork, often require the inclusion of a wax which enables the polyester resin to cure properly. This wax migrates to the exterior surface of the fiberglass product and it is virtually impossible to remove it. Attempts to sand the wax away usually drives it deeper into the product. Heretofore, it was necessary to wet sand or scour the ends of the fiberglass ducts with items such as alcohols or acetones to remove the wax. Once removed, there was still an enamel-like finish to contend with and this could only be removed by use of a grinder or sander at the jobsite. This procedure created several problems. Not only was it labor intensive and costly, but also the sanded FRP particles tended to get into clothes, nostrils, eyes, and ears of workers if unprotected.
In addition to the aforesaid sanding problems, prior procedures for field connections of FRP duct sections required that the same resin used to produce the fiberglass reinforced plastic duct product should also be used in the bonding process in the field. This required that catalysts (Part B) be mixed at the job site with the base resin (Part A). In the case of polyesters, often dangerous materials such as methyl ethyl ketone peroxide are used; in the case of epoxies, aliphatic or aromatic amines (Part B) are used, and in the case of various types of phenols or modified phenols, formaldehyde sources (Part B) must be added. The workmen in the field were required to carefully measure the weight ratios of the resin components (Parts A and B) prior to mixing them, and this created a nuisance factor and was prone to error. Thereafter, glass fabric material were saturated with the mixed resin (Part A & B) and applied in various layers with proper "rollout" between each layer. This rollout was generally done with fiberglass rollers which forced the air out of the glass/resin laminate. When joining sections in the field in this prior art manner, two sections (duct to duct or duct to fitting) were required to be held in close proximity to each other and not permitted to move. This field joint, after the application of the glass and resin, also could not be moved in any way until the resin itself had completed its cure, i.e. hardened.
Those who have worked in the field of FRP ductwork have long sought to develop an effective and efficient method and means for eliminating the above problems and complications of prior field duct connections. In certain industries such as the semiconductor or pharmaceutical industry, ductwork is generally installed in areas that are super clean, known as clean rooms. In these industries, great concern is made about even very small particles such as two to five microns in size, which are considered to be very large contaminates in the semiconductor industry. Construction of these types of facilities (semiconductor, pharmaceutical) often consists of installing miles of fiberglass reinforced plastic duct for the main ducts. Fiberglass reinforced plastics is a preferred material because of its chemical resistance and comparative low cost. However, building and fire codes often require that this non-metallic ductwork be "fire resistant" as well as corrosion resistant, and the duct joints made in the field at the time of installation must have comparable fire resistance to the ductwork itself. Thus, another stringent requirement for FRP ductwork connections is that they maintain a high fire-resistant quality.
With previous attempts to provide FRP duct coupling devices another serious problem arose involving the use of gasketing elements that were necessary to provide leak-proof connections. When conventional off-the-shelf non-fire retardant plasticized gasketing material was used, the high heat of a typical duct fire caused commonly compounded gasket materials to melt and become liquid, thereby functioning as a lubricant. When this occurred the resulting lubrication effect and also the reduction in or shrinking of the overall duct structure due to the intense heat caused duct sections to slide apart. Thus, such prior art duct coupling structures failed to pass such functional tests as the Factory Mutual Test. This problem has been solved by the present invention which provides duct coupling devices that maintain a high degree of structural integrity even under severe duct fire conditions.
In view of the foregoing, it is therefore one object of the present invention to provide a novel method of making field joints, i.e., duct connections or duct to fitting connections in FRP duct systems which eliminates the necessity of sanding, the used of glass mat or the use of odor causing resins within the clean room facilities presented herein.
Another object of the invention is to provide connection means for duct sections or duct sections and fittings that have a high degree of fire resistance when tested under accepted industry standards.
Another object of the invention is to provide a coupling assembly for connecting abutting duct sections that is leak-proof, strong and durable and which will maintain a high degree of structural integrity even under severe internal duct fire conditions.
Still another object of the invention is to provide a mechanical device for connecting FRP duct sections which is easy to install at the job-site with relatively unskilled labor and without special tools and yet which is leak-proof, long lasting and resistant to chemical corrosion.
A more specific object of the invention is to provide a coupling of abutting FRP duct section having an outer band that will automatically tighten around the duct point if the diameter of the joint should decrease due to shrinkage of the coupling gasket during a fire.
Another object of the invention is to provide an FRP duct coupling having gasket components which will swell and increase in volume in response to increased temperature during an internal duct fire so as to help maintain the coupling firmly in place on the duct joint.